This invention relates to resonators, also known as solid horns, concentrators, tools, amplitude or velocity transformers, which are used to couple mechanical vibrations in the sonic or ultrasonic frequency range from a source of vibrations to a workpiece. Resonators of this type are used extensively in connection with ultrasonic apparatus employed for welding thermoplastic workpieces, for welding metallic workpieces, for producing emulsions, etc.
Resonators of the type indicated above are metallic sections dimensioned to be resonant as half wavelength resonators when driven at a predetermined frequency of sound travelling longitudinally therethrough from an input end to an opposite output end. The resonators, depending upon the end use requirements, are made most commonly of aluminum and titanium, less frequently of steel and Monel. A rather comprehensive description of the different types of resonators and their design can be found in "Ultrasonic Engineering" (book) by Julian R. Frederick, John Wiley & Sons, Inc., New York, N.Y. (1965), pp 87-103.
When sealing or bonding superposed layers of textile material by ultrasonic energy, so-called bar resonators are used. Resonators of this type have a rectangular cross section and, most commonly, include a cross-sectional reduction in the nodal zone of the resonator in order to provide increased motional amplitude at the output end when compared with the input end at which the mechanical vibrations are applied. In this manner the resonator, resembling a blade, serves not only to couple vibrations from a source to a workpiece, but also operates as a mechanical amplifier for the vibratory amplitude.
A typical blade-type resonator is shown in U.S. Pat. No. 3,113,225 entitled "Ultrasonic Vibration Generator", issued to C. Kleesattel et al dated Dec. 3, 1963 and the use of such a resonator in connection with sealing superposed textile material by ultrasonic energy is shown in U.S. Pat. No. 3,733,238 dated May 15, 1973 entitled "Apparatus for Vibration Welding of Sheet Material" issued to D. D. Long et al. A similar ultrasonic sealing arrangement is disclosed in U.S. Pat. No. 3,562,041 issued Feb. 9, 1971 entitled "Method and Apparatus for the Ultrasonic Joining of Materials According to a Pattern" in the name of C. A. Robertson which patent reveals the manufacture of shirt sleeve cuffs and similar articles.
The problem heretofore encountered is that the frontal surface of these horns, which may have a width from 4 inch (100 mm) to 8 inch (200 mm) or more, exhibits an uneven vibrational amplitude. Typically, the vibrational amplitude is of the desired amplitude at the center region of the resonator, but decreases significantly toward the lateral edges. When sealing plastic film and textile materials a relatively high motional amplitude is required, typically 0.003 to 0.005 inch (0.08 to 0.13 mm) peak-to-peak displacement, and since the ultrasonic sealing when operating on soft or flexible material is confined to the area directly underneath the resonator, welding of the material may be satisfactory underneath the center portion of the resonator, but unsatisfactory toward the lateral edges of the resonator.
In the past attempts have been made to design a resonator of rectangular configuration, notably a blade type resonator, which exhibits a substantially uniform motional output along the entire output surface. One of the earliest efforts has been the provision of slots which traverse the nodal region of the resonator to thereby break the Poisson coupling, see Kleesattel supra. Further improvements became necessary which are exemplified by P. H. Davis, U.S. Pat. No. 4,131,505 dated Dec. 26, 1978 entitled "Ultrasonic Horn" and German patent publication Ser. No. P 23 43 605 filed Aug. 29, 1973 by Mecasonic S. A.
The Davis patent discloses the use of a peripheral groove in the lower (output) portion of the resonator and applies this groove concept to solid round and rectangular resonators. The Mecasonic patent publication specifically deals with blade-shaped resonators and adds tuned half wavelength resonators at the lateral portions of the resonator in order to obtain substantially uniform motional output along the entire resonator output surface from the central portion to the edges. Assuming a frequency of 20 kHz and material such as aluminum, steel or titanium, a half wavelength resonator is about 51/4 inch (134 mm) long. Hence, considerable weight and height is added to the normally quite heavy resonator by adding two additional half wavelength resonators in a "piggy-back" manner to the input surface of the resonator, see FIG. 4 of the Mecasonic publication.